1. Field of the Invention
The present invention relates to a reversible thermosensitive recording medium, and more particularly to a reversible thermosensitive recording medium utilizing a coloring reaction of an electron donating coloring compound with an electron accepting compound. In addition, the present invention relates to an image forming and erasing method using the reversible thermosensitive recording medium.
2. Discussion of the Background
In view of environmental problems such as increase of dust and destruction of forests, considerable attention is currently placed on reversible thermosensitive recording media which can reversibly record and erase an image many times. Therefore various reversible thermosensitive recording media have been proposed, and some of the recording media are marketed now.
For example, Japanese Laid-Open Patent Publications Nos. (hereinafter referred to as JOP) 63-107584 and 4-78573 have disclosed polymer-type reversible thermosensitive recording media utilizing a physical change, in which a transparent state and an opaque state are reversibly achieved upon application of heat thereto.
In addition, JOPs 60-193691 and 2-188293 have disclosed dye-type reversible thermosensitive recording media utilizing a chemical change, in which a combination of gallic acid with phloroglucinol is used, or a salt of a higher aliphatic amine and bis(hydroxyphenyl)acetic acid or gallic acid is used as a color developer.
Some of the present inventors and other inventors propose a reversible thermosensitive recording medium using a coloring agent and a color developer in JOP 5-124360, etc. Namely, by using a combination of a specific color developer (i.e., an electron accepting compound) with a specific coloring agent (i.e., a leuco dye, or an electron donating coloring compound), a color image can be easily formed and erased reversibly when properly controlling heating and cooling conditions. The reversible thermosensitive recording medium can reversibly achieve a colored state and a non-colored state many times, and the colored state and non-colored state can be stably maintained at room temperature.
Such a reversible thermosensitive recording medium has been improved as disclosed in JOP 6-210954, and is now used for, for example, point cards in which information of the amounts of points given to a user proportionally to the total purchase amounts in a shop is displayed.
Reversible thermosensitive recording media including a combination of a color developer and a leuco dye have a drawback in that when the recording media are exposed to light for a long period of time, the media tend to color blown and the color cannot be erased. The reason is considered to be that the molecular structure of the leuco dye changes due to irradiation of light, resulting in formation of irreversible colored materials.
Conventional irreversible thermosensitive recording media including a combination of a leuco dye and a color developer also have such a coloring problem. In attempting to solve the problem, an ultraviolet absorbent is typically included in the irreversible thermosensitive recording media to prevent the leuco dye being exposed to ultraviolet light.
For example, JOP 62-48585 discloses an irreversible thermosensitive recording medium in which an intermediate layer including a low molecular weight ultraviolet absorbent is formed between a recording layer and a protective layer.
The present inventors find that when this technique is applied to a reversible thermosensitive recording medium, a problem which occurs is that the coloring/erasing properties of the recording medium are changed because the main component of the ultraviolet absorbent migrates to the recording layer. In addition, it is also found that big problems tend to occur such that the ultraviolet absorbent migrates to the surface of the protective layer, resulting in deterioration of ultraviolet light absorbability of the recording medium, and image qualities deteriorate because the migrated ultraviolet absorbent adheres to a thermal printhead serving as an image writing device.
JOP 7-68937 discloses a reversible thermosensitive recording medium including a protective layer which includes a microencapsulated ultraviolet absorbent which is liquid at room temperature. This microencapsulated ultraviolet absorbent is used to prevent the ultraviolet absorbent from bleeding out. However, the protective layer has a drawback of having poor film strength because of including a liquid phase, and thereby the protective layer deforms after long repeated use. In addition, if the microcapsule is damaged, various problems occur such that the coloring/erasing properties deteriorate; a thermal printhead is contaminated, resulting in deterioration of image qualities; and users are contaminated. Therefore, this method can fully solve the coloring problem.
JOP 10-100541 discloses a reversible thermosensitive recording medium in which a particulate inorganic pigment having ultraviolet-ray masking ability is included in an intermediate layer or a protective layer. Since the particulate inorganic pigment has a controlled particle size, the resultant layer has ultraviolet scattering ability. Therefore the layer has a combination of ultraviolet absorbing ability and ultraviolet scattering ability, and thereby the recording layer is prevented from being exposed to ultraviolet rays.
However, in this method a large amount of a very fine inorganic pigment has to be included in the intermediate or protective layer. Therefore, the layer is brittle. When a reversible thermosensitive recording medium including such a brittle layer is processed so as to be a sheet or a card, a problem such that the edges of the sheet or card have burrs. In addition, such reversible thermosensitive recording medium has high manufacturing costs because the fine inorganic pigment is very expensive. Further, the properties of such a recording medium largely change depending on the dispersion of the very fine inorganic pigment, and therefore problems with image qualities often occur.
In addition, JOPs 8-224960 and 9-207437 have disclosed reversible thermosensitive recording media which include a protective layer including a polymer having ultraviolet absorbability.
In general, the protective layer of a reversible thermosensitive recording medium has to have high heat resistance and durability, and therefore a resin layer crosslinked upon application of heat or ultraviolet rays is typically used as the protective layer. When a polymer which has ultraviolet absorbability and which is not crosslinked is used for the protective layer, the resultant recording medium has poor heat resistance and durability.
Even when a combination of a polymer having ultraviolet absorbability with a crosslinkable resin is used to form a protective layer in which the polymer is fixed by the crosslinked resin, the film strength of the protective layer deteriorates similarly to the protective layer including a microcapsule mentioned above, and thereby the non-crosslinked polymer having ultraviolet absorbability is firstly damaged and then the surface of the recording medium deforms after long repeated use. Therefore, bad image formation/erasure problems occur, resulting in shortage of the life of the recording medium.
As can be understood from the above-description, a reversible thermosensitive recording medium having a good combination of light resistance and durability has not yet been developed.
In JOPs 13-180188 and 13-180116, the present inventors have proposed a reversible thermosensitive recording medium which includes a combination of an electron donating coloring compound and an electron accepting compound and which is used for forming a temporary document such as documents for a meeting.
Such a document-use reversible thermosensitive recording medium has a function in which plural images stored in a computer as digital information are illustrated in plural sheets of the recording medium, which a user can see while comparing the plural images. Therefore the document-use reversible thermosensitive recording medium is required to be easy to handle. Namely, the medium is required to be freely arranged on a table; to be rearranged; to be seen while a user picks up it; and be able to rewrite an image, if desired.
Thus, the document-use reversible thermosensitive recording medium is used under various conditions, and often has a chance of being exposed to light, which is different from the card-use reversible thermosensitive recording medium.
There is no proposal for a document-use reversible thermosensitive recording medium having an excellent combination of light resistance and durability. Namely it is a new issue to be addressed.
In addition, if the document-use reversible thermosensitive recording medium preferably has a good writing ability such that an image can be easily written by a general writing material such as markers on the surface of the recording medium and the written image is also easily erased, the recording medium will be able to be widely used. However, reversible thermosensitive recording media has a drawback in that when images are written on the surface thereof using a general writing material, the recording media cannot be used thereafter because the images are hardly erased.
JOP 7-113055 discloses a heat-erasable ink by which a heat-erasable image can be written on the surface of reversible thermosensitive recording media. However, since the constituents of the ink image written by a writing material on reversible thermosensitive recording media remain on the surface of the recording media even after the image is non-colored upon application of heat. Therefore, when the recording media are repeatedly used while being heated by a thermal printhead, the recording media tend to produce an undesired image because the remaining constituents adhere to the thermal printhead.
JOPs 10-100536 and 5-286258 have disclosed display media in which a reversible thermosensitive recording layer is formed on one side of a transparent support and an image can be written by a writing material on the other side of the support. However, the recording media have drawbacks in that the support is limited to transparent materials and the media have poor visibility (i.e., poor visual property) because a user looks a recorded image through the support.
Because of these reasons, a need newly exists for a document-use reversible thermosensitive recording medium having a surface having good writing/erasing properties and capable of reversibly forming and erasing an image many times while having good light resistance.
Accordingly, an object of the present invention is to provide a reversible thermosensitive recording medium which can reversibly form and erase an good image without causing deformation even after long repeated use while having a good light resistance and which can be used for document applications as well as card applications.
Another object of the present invention is to provide a reversible thermosensitive recording medium having a surface on which an image can be written by a writing material and the image can be erased by an eraser such as non-woven cloths, papers, sponges, rubbers, cloths, etc. without a residue thereon while the image has a fixability so as not to be easily erased when contacted with other documents.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by a reversible thermosensitive recording medium having a substrate; a recording layer on the substrate, which includes an electron donating coloring compound and an electron accepting compound, wherein the recording layer achieves a colored state when heated at a temperature not lower than an image forming temperature and then cooled at a cooling speed, and the recording layer in the colored state achieves a non-colored state when heated at a temperature lower than the image forming temperature and not lower than an image erasing temperature or when heated at a temperature not lower than the image forming temperature and then cooled at a cooling speed relatively slow compared to the first-mentioned cooling speed; and a crosslinked polymer layer having an ultraviolet absorbing structure.
The recording layer may serve as the crosslinked polymer layer, however, the crosslinked polymer layer is preferably formed on the recording layer. In addition, the crosslinked polymer layer preferably has a viscoelastic logarithmic decrement property such that a peak temperature is not lower than 100xc2x0 C., and a logarithmic decrement at the peak temperature is not greater than 0.3. Alternatively, the crosslinked polymer layer may have a peak temperature not lower than 150xc2x0 C., and a logarithmic decrement at the peak temperature is not greater than 0.6.
The ultraviolet absorbing structure is at least one of a benzotriazol structure and a benzophenone structure.
In addition, it is preferable that the polymer has a hydroxyl group and is crosslinked using an isocyanate compound as a hardener.
Further, the surface of the reversible thermosensitive recording medium has a dynamic receding contact angle against water of from 75xc2x0 to 100xc2x0. The crosslinked polymer layer is preferably the surface layer, and a silicone-modified polymer is preferably included in the crosslinked polymer layer, which is preferably crosslinked with the polymer having an ultraviolet absorbing structure. The surface layer preferably includes a filler. It is preferable that the filler is coated with a calcium compound and has an average particle diameter of from 0.2 to 2.0 xcexcm.
The substrate is preferably a paper.
The recording medium may be a card, a sheet or a roll, and may have a print layer and/or an information storing portion such as magnetic recording layers.
In another aspect of the present invention, a reversible thermal image recording method is provided which includes the steps of:
providing the reversible thermosensitive recording material of the present invention; and
imagewise heating the recording material at a temperature not lower than an image forming temperature and then cooled rapidly to form an image in the recording layer.
In yet another aspect of the present invention, a reversible thermal image erasing method is provided which includes the steps of:
providing the reversible thermosensitive recording material of the present invention; and
heating the recording material at a temperature lower than an image forming temperature such that the recording layer achieves a non-colored state.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.